In order to achieve a satisfactory operation of a resonant converter, it is important that switches for generating an alternating current are switched on and off at the right instant. A frequency at which the switches are operated defines a mode of operation of the converter. If the frequency is sufficiently high, the energy converter operates in a regular inductive mode.
The converter operates in a near-capacitive mode when the switching frequency of the switches, and hence the frequency of the alternating current through the primary side of the transformer is decreased to a point where the alternating current is at least almost in phase with a alternating current at a bridge node.
Generally, it is desirable that the energy converter operates in the inductive mode. To this end, it is important that the non-overlap time is chosen to be sufficiently long to prevent hard switching, resulting in switching losses. However, the non-overlap time is bound to a maximum because hard switching also occurs in the case of a too long overlap time so that switching losses occur.
In US patent application 2001/0036090 a control algorithm of the switches is disclosed where the switch turn on moment is determined in such a way that the switch is turned on close to the top (high side switch turn on) or valley (low side switch turn on) of a half bridge switching node. In this way minimum voltage drop occurs over the switch at the moment that it is turned on, at a given current in the transformers primary side, providing minimum switching losses. The turn off moment is determined in a known manner. A known manner may e.g. be increasing the switching frequency. The amplitude of hard switching is then measured and used to regulate the frequency to just that values that gives the desired amplitude of hard switching. A disadvantage of this method is it may be difficult to stabilize the converter.